The invention relates to digital television receivers for vestigial-sideband (VSB) digital television (DTV) signals and, more particularly, to the portions of such receivers used for recovering baseband symbol coding proceeding from intermediate-frequency signals.
Bandpass trackers for VSB DTV signal receivers are described by C. B. Patel and the inventor in U.S. Pat. No. 5,479,449 issued Dec. 26, 1995 and entitled xe2x80x9cDIGITAL VSB DETECTOR WITH BANDPASS PHASE TRACKER, AS FOR INCLUSION IN AN HDTV RECEIVERxe2x80x9d. In U.S. Pat. No. 5,479,449 VSB DTV signal in a final intermediate-frequency band is digitized before being synchrodyned to baseband in a digital demodulation procedure designed to reproduce the transmitted baseband symbol code. U.S. Pat. No. 5,479,449 describes the use of narrow bandpass filters to convert the digitized VSB DTV signal in that final I-F band to a digital narrow-band double-sideband amplitude-modulation (DSB AM) signal. The narrow-band DSB AM signal is synchrodyned to baseband in the digital regime for recovering an automatic-frequency-and-phase-control (AFPC) signal for a local oscillator that generates local oscillations used in detecting the VSB amplitude modulation to generate the final I-F signal. U.S. Pat. No. 5,479,449 also describes alternative bandpass trackers for VSB DTV signal receivers which do not use a narrow bandpass filter to convert the digitized VSB DTV signal to a digital narrow-band DSB AM signal, but instead extract the AFPC signal for the local oscillator from the response of a narrowband lowpass filter to the quadrature-phase component of the result of complex synchrodyning of the VSB DTV signal to baseband. This alternative type of bandpass tracker has been favored because the narrowband filtering of the AFPC signal can be carried out in the analog regime, without employing a digital filter of considerable complexity.
The digital synchrodyning procedure used in U.S. Pat. No. 5,479,449 for the in-phase demodulation of the VSB DTV signal to recover baseband symbol code employs a complex digital multiplier. The digital synchrodyning procedure used in U.S. Pat. No. 5,479,449 for the quadrature-phase demodulation of the VSB DTV signal to develop the AFPC signal employs another complex digital multiplier. These complex digital multipliers each receive a complex multiplicand signal comprised of the digitized final I-F signal and its Hilbert transform, as developed by digital filtering. These complex digital multipliers receive respective complex multiplier signals from a read-only memory (ROM) addressed from a sample counter counting at a rate in a prescribed ratio to the VSB DTV signal baud rate, which complex multiplier signals are orthogonal to each other.
The design of digital filtering to generate an accurate Hilbert transform response over a five to six megahertz bandwidth is a formidable problem, particularly if the lower end of the passband is closer to zero-frequency than one megahertz or so. Because of this, bandpass trackers for VSB DTV signal receivers have used a final I-F signal with a carrier above one megahertz with a full-band sideband extending upward therefrom in frequency and a vestigial sideband extending downward therefrom in frequency. Alternatively, bandpass trackers for VSB DTV signal receivers have used a final I-F signal with a carrier more than a megahertz above the 5.38 megahertz one-half baud rate with a full-band sideband extending downward therefrom in frequency and a vestigial sideband extending upward therefrom in frequency.
A complex digital multiplier comprises four component four-quadrant digital multipliers, a digital subtractor and a digital adder. Practically speaking, one-half of such a complex digital multiplier is a necessity in the synchrodyning procedure used for developing AFPC signal for the local oscillator that generates local oscillations used in detecting the VSB amplitude modulation to generate the final I-F signal. Otherwise, the AFPC loop exhibits low gain when carrier phasing is at the edges of a pull-in range, that can lead to an undesirable lack of stable phase lock in this feedback loop. The controlled local oscillator preferably has exceptional frequency stability in the absence of a fed-back AFPC error signal, which stability is achievable using crystal frequency stabilization. If the frequency of the local oscillator is far enough from carrier frequency to fall within the narrow spectral width of the vestigial sideband, the energy required for pull-in and lock-in of the AFPC loop is very likely to be unavailable. The bandwidth of the AFPC loop is designed to be quite narrow, almost certainly less than xc2x1300 kilohertz at most, generally much less.
The design of digital filtering to generate an accurate Hilbert transform response over this limited frequency range can be a substantially simpler problem than the design of digital filtering to generate an accurate Hilbert transform response over a five to six megahertz bandwidth, the inventor observes. This is especially so if the VSB DTV carrier as translated to the final I-F band is 5.38 MHz or more.
The inventor further observes that if the AFPC loop carries out its appointed function, there is no need for a complex digital multiplier in the synchrodyning procedure used for demodulating the VSB DTV signal to reproduce the transmitted baseband symbol code. The AFPC of the local oscillator permits a real-only digital carrier to multiply a real-only final I-F signal in a single four-quadrant digital multiplier. That is, there is no need for two component four-quadrant digital multipliers and a digital adder; and there is no need for Hilbert transform filtering to generate an imaginary component of digital I-F signal for demodulating the VSB DTV signal to reproduce the transmitted baseband symbol code. This provides for considerable simplification in the bandpass tracker structure.
The invention is embodied in a receiver for a baseband symbol code that is designed to have a spectrum reaching at least substantially to zero-frequency and that is transmitted by vestigial-sideband amplitude modulation of a suppressed carrier wave. The receiver includes a local oscillator for generating local oscillations used in detecting the vestigial-sideband amplitude modulation. A mixer is included in the receiver to heterodoxy the local oscillations with vestigial-sideband amplitude modulation of a suppressed carrier wave to generate a final intermediate-frequency signal. The receiver includes an analog-to-digital converter to digitize the final intermediate-frequency signal. The receiver is characterized by real-only digital synchrodyne circuitry for synchrodyning a real digital carrier with the digitized final intermediate-frequency signal to generate an in-phase synchronous detection signal.
In preferred embodiments of the invention the local oscillator is one which has the frequency and phase of its local oscillations subject to automatic-frequency-and-phase-control (AFPC). Digital filtering is included in such a preferred receiver for supplying a complex frequency response to the digitized final intermediate-frequency signal, with improved symmetry of channel response in a narrow-frequency band including the pilot carrier signal. Further digital synchrodyne circuitry is included in such a preferred receiver for multiplying the complex frequency response to the digitized final intermediate-frequency signal by a complex digital carrier in half a complex multiplication operation that generates a quadrature-phase synchronous detection signal. Automatic-frequency-and-phase-control circuitry responsive to an error signal extracted from the quadrature-phase synchronous detection signal completes an automatic frequency and phase control (AFPC) loop in such a preferred receiver for controlling the frequency and phase of the controlled local oscillator so as to minimize the error signal.